(1) Field of the Invention
The present invention relates to an SOI (silicon on insulator) substrate, and more particularly to an SOI substrate having enhanced gettering effects.
(2) Description of the Related Art
In a highly integrated semiconductor device, the device characteristics are markedly deteriorated by impurities or faults inherent in materials and also by faults induced or introduced during the fabrication process, and especially by contamination caused by heavy metal impurities. Therefore, it is necessary for the device to have gettering effects in order to eliminate heavy metals or faults from an element area of the device.
A first example of a conventional SOI substrate having a gettering effect with respect to heavy metals or faults has been disclosed in Japanese Patent Application Kokai Publication No. Hei 2-237121. In this example, as shown in FIG. 1, a buried silicon oxide film 22 is partially formed in an upper surface of a single crystal silicon substrate 21 by a selective oxidation process and, after being flattened or planarized, this is bonded onto the overlying single crystal silicon substrate 23. This structure can achieve intrinsic gettering effects in an IG (intrinsic getter) fault layer 21A of the underlying single crystal silicon substrate 21.
A second conventional example has been disclosed in Japanese Patent Application Kokai Publication No. Hei 3-132055. In this example, as shown in FIG. 2, a silicon oxide film 22A is formed on a first single crystal silicon substrate 21, on which a buffer layer 24 such as an amorphous silicon film or polycrystalline silicon (also called polysilicon) film having gettering capabilities and film strain relaxing capabilities is provided, and then a second single crystalline silicon substrate 23A is formed thereon.
A third conventional example which, although not of an SOI substrate, includes a gettering means also used in element isolation, has been disclosed in Japanese Patent Application Kokai Publication No. Sho 63-271941. In this example, as shown in FIG. 3, an N-type epitaxial layer 26 is formed on a single crystal silicon substrate 21 having therein an N-type buried diffusion layer 25, an element isolation region is patterned to form a trench which is filled with a polycrystalline silicon film 24A followed by a boron diffusion therein. The polycrystalline silicon film 24A is used as an isolation element and at the same time as a gettering site for heavy metal contamination.
In the first and the third example shown in FIG. 1 and FIG. 3, respectively, since the isolation is incomplete with respect to the underlying single crystal silicon substrate 21, it is not possible to sufficiently reduce soft-errors caused by carriers originating as a result of .alpha. radiation. Moreover, in the third example, since boron (B) of the polycrystalline silicon film 24A diffuses to the device formation region during the device fabrication and in thermal treatment during subsequent device formation, the possibility of realizing higher device density is prevented.
In the second example, since the polycrystalline silicon film 24, which becomes the gettering site, is provided on the entire surface of the silicon oxide film 22A, warping of a substrate occurs thereby causing strain and faults in the substrate. The polycrystalline silicon film 24 is weak in its gettering capability with respect to iron (Fe) and, moreover, the gettering site thereof is located close to the device formation region. A disadvantage therefore is that the substrate is likely to be affected by the re-diffusion of heavy metals, having previously once undergone gettering, into the device formation region.